1. Field of the Invention
This invention relates to a view finder optical system and more particularly to a view finder optical system highly suited for a single-lens reflex camera and for a so-called electronic camera of a TTL optical system using a camera tube or a solid-state image sensor such as a CCD.
2. Description of the Prior Art
Heretofore, single-lens reflex cameras of the kind using a pentagonal roof type prism for 35 mm film have advanced to a great extent as this kind of camera is most suited for expansion of camera systems. FIG. 1 of the accompanying drawings schematically shows the view finder optical system of a typical single-lens reflex camera. Referring to FIG. 1, the illustration includes a total reflection mirror 101, a shutter unit 102, a film surface 103, a focusing screw 104, a pentagonal roof type prism 105, an eye-piece 106 and a pupil 107 for observation. The view finder optical system shown in FIG. 1 excels in optical performance having more than 90% field ratio which means a ratio of an image plane to be photographed on the film surface to the image of an object observable through the view finder optical system and also having a field magnification .gamma. of at least 0.8 with a standard lens mounted on the camera. It is a feature of this view finder optical system that it permits the whole of the camera to be compactly arranged.
In cases where the above-stated pentagonal roof type prism 105 is used for the view finder optical system of an electronic camera using an image sensor such as a CCD, it becomes difficult to obtain the same field ratio and the same field magnification as those of the conventional single-lens reflex camera. Furthermore, in that event, it becomes difficult to compactly arrange the whole apparatus. The reasons for these difficulties are as described below:
(i) The effective image plane of an image sensor, for example, of 2/3 inch is smaller than the 35 mm film and is about 1/4 of the latter in the ratio of diagonal length. Therefore, with the conventional pentagonal roof type prism 105 employed, the optical path length becomes too long to obtain a high field ratio and a high degree of field magnification.
(ii) An electrical processing circuit requires a large space behind the image sensor. This causes a distance between the image plane of the photo-taking lens and the hindmost end of the camera. Then, the pupil 107 position of the view finder optical system must be arranged farther in the rear part of the camera. As a result of this, it becomes difficult to obtain a high field ratio and a high degree of field magnification.
(iii) The photo-taking lens is arranged in a telecentric structure for color separation. This causes an effective light flux from the photo-taking lens to spread to a greater extent at a part where the view finder optical path is parted. Therefore, the size of the reflection mirror 101 becomes larger.
(iv) The optical members such as a low-pass filter, an infrared ray cut filter, protection glass, etc. which are disposed in front of the image sensor require a large space. This necessitates a long distance between the view finder optical path splitting point and the image sensing plane. As a result, the size of the camera as a whole increases.
Next, an example of a view finder optical system which is intended to attain at least a 90% field ratio by using a conventional pentagonal roof type prism 210 for an electronic camera is arranged as shown in FIG. 2. The illustration includes a photo-taking lens 200; a split unit 201 which is arranged to split the photo-taking optical path to obtain an optical path leading to the view finder optical system; a low-path filter 202; a shutter unit 203; an image sensing surface 204 of an image sensor; an image sensor package 205 provided with a protection glass which has an infrared cutting effect and is disposed in front of the sensor package; a view finder optical system unit 206 which includes an erecting image system; a focusing screen; an electrical processing circuit unit 208 which is arranged to electrically process an image sensing signal; and a pupil 209 for observation.
Generally, the larger the field magnification .gamma., the more easily the view finder image is observable. The field magnification .gamma. can be expressed as .gamma.=f.theta./fe, where f.theta. represents the standard focal length of the photo-taking lens 200 and fe the focal length of the eye-piece 211. Since the focal length of the standard lens 200 is almost unvarying, in order to make the field magnification .gamma. larger, the focal length fe of the eye-piece 211 must be shortened.
The eye-piece 211 is arranged to have its front side focal point close to the finder image forming plane of the view finder optical system 206. Therefore, in order to increase the field magnification .gamma., the optical path length of the optical system 206 arranged to obtain a positive erecting image between the focusing screen and the eye-piece 211 must be shortened.
Assuming that the focal length f.theta. is arranged to be f.theta.=12.5 mm to correspond to the standard lens 200 for a 2/3 inch image sensor and the field magnification .gamma. to be .gamma.=0.6, the focal length fe becomes fe=20.8 mm.
Further, in order to obtain a high field ratio, it is necessary to have a focusing screen which is of about the same size as that of the effective image sensing image plane and an erecting image system which is large enough for forming some optical path that permits adequate observation of the focusing screen. In the arrangement shown in FIG. 2, therefore, in order to obtain at least a 90% field ratio and at least a field magnification of .gamma.=0.6, the following arrangement is necessary: First, the optical path length from the focusing screen to the principal point in front of the eye-piece 211 is arranged to be 20.8 mm which is approximately equal to the focal length of the eye-piece 211. In addition to that, light from the focusing screen must be arranged to be sufficiently incident on the eye-piece 211. It is possible to find a pentagonal roof type prism capable of meeting such a requirement. However, the pentagonal roof type prism 210, as shown in FIG. 2, must be disposed in a foremost part within the view finder unit 206 while the eye-piece 211 must be arranged to be adjacent to the exit plane of the pentagonal roof type prism 210. Meanwhile, the pupil 209 for observation must be arranged to be far away from the eye-piece 211 and positioned behind the rear end of the camera. However, since a principal ray is incident on the eye-piece 211 in parallel with the optical axis, the pupil 209 position is located at a distance about equal to the focal length from the second principal point behind the eye-piece 211. Therefore, as shown in FIG. 2, in order to have the eye-piece 211 and the position of the pupil 209 separated at a great distance from each other, the distance between the first and second principal points of the eye-piece 211 must be arranged to be long. Such an arrangement is extremely difficult. In the case of the electronic camera or the like having a relatively small effective image or picture plane, therefore, use of the pentagonal roof type prism 210 for the view finder optical system 206 makes it extremely difficult to obtain a high field ratio and a high field magnification because of optical performance.
Meanwhile, the conventional view finder optical system 206 using the pentagonal roof type prism 210 has some space left over to permit information display light to be readily introduced from behind the focusing screen. However, with a large space used along the view finder optical axis for insertion of a light source and a display element for information displayed between the focusing screen and the plane of incidence of the prism 210 results in a longer length of the optical path between the focusing screen and the eye-piece 211. The longer optical path then results in a lower field magnification and is not desirable.
Further, if the focal length of the eye-piece 211 is shortened for the purpose of increasing the field magnification, even a slight discrepancy between the focusing screen and an information display surface (the position of the light source or the display element) results in a great deviation of diopter.
While the above-stated problems result from the optical arrangement, there are the following problems in terms of manufacturing and assembling processes: For example, if the relative positions of the focusing screen and the view finder prism 210 deviate during the manufacture, the position of a field image on the final exit plane of the prism 210 deviates and thus the image is either eclipsed or becomes squinted.
Japanese Laid-Open Utility Model Application No. SHO 56-53372 has disclosed a view finder which resembles in appearance the view finder optical system of an embodiment of this invention which will be described later herein. The view finder disclosed, however, essentially differs from that of this invention as it is designed for the use of a much larger film than the 35 mm silver halide film. Two marginal rays have a convergent inclination. This is because an eye-piece of a long focal length can be used for a camera which has a large picture plane. In such a case, a sufficient observation pupil distance can be secured even if the marginal rays are convergent. Therefore, it is not necessary to use a prism which is large relative to the size of the camera body. If the size of the picture plane is small, however, in order to make the focal length of the eye-piece shorter for the purpose of obtaining a high magnification, the marginal rays must be arranged to be as parallel as possible with each other. Then, the arrangement disclosed by the above-stated Japanese utility model would result in a large prism.
Meanwhile, Japanese Laid-Open Utility Model Application No. SHO 49-117841 has disclosed a view finder arrangement in FIG. 1 thereof. In this case, the second reflection plane of a first prism seems to require a specular treatment as it fails to satisfy a condition required for total reflection. Then, since the incident area and the reflection area of a light flux cannot be arranged to overlap each other, the arrangement disclosed in FIG. 1 inevitably causes an increase in the size of the first prism. In another arrangement disclosed in FIG. 2, the light flux can be totally reflected. However, since the incidence plane is slanted in this case, this arrangement necessitates some additional arrangement to correct astigmatism or distortion at some other part.
A first object of this invention is to provide a view finder optical system which has a high field ratio and a high field magnification and is highly suited for a camera having a relatively small photo-taking image plane.
A more specific object of this invention is to provide a compact view finder optical system having at least 90% field ratio and about a 0.6 field magnification.
Another object of this invention is to display information in the neighborhood of a view finder field.
A further object of this invention is to provide an optical element which can be precisely mounted in a predetermined position on a framework within a camera.
A still further object of this invention is to prevent a ghost image from occurring within a view finder field.